1. Field of the Invention
This invention relates to methods and apparatus for obtaining accurate measurements. In particular, the invention is directed to a method for improving the resolution of a measured parameter that is represented by a substantially periodic electrical signal.
2. Description of the Prior Art
It is well known to generate substantially periodic analog electrical signals having a period T which is proportional to a measured parameter. Such a technique has been used to continuously measure parameters such as speed with optical tachometers and to measure the diameter of strand material by directing light from a laser at the strand causing a diffraction fringe pattern to impinge on a light detecting diode array. The diode array is periodically scanned resulting in a continuous, substantially sinusoidal output signal having a period, T, which is proportional to the strand diameter, such a method is limited to counting signal peaks resulting in a signal resolution of T. As a result of such limited resolution, when the parameter being measured changes, causing changes in the detected frequency, such a scheme cannot sense, very quickly, incremental changes in the measured parameter.
One method of providing higher resolution and a more sensitive indication of changes in the measured parameter is to double the detected frequency. U.S. Pat. No. 3,965,430 to Brandt which issued on June 22, 1976 describes a system for digitizing a signal from an optical tachometer which doubles the frequency of the sinusoidal input signal by forwarding a square wave representation of the original sinusoidal signal to a pair of pulse generators connected in parallel and which are arranged to trigger on opposite polarity slopes of the square wave signal. Outputs from each generator are forwarded to an "OR" gate resulting in a pulsating output frequency that is double that of the original input signal. The resolution of such a scheme is T/2.
Such a method clearly improves the resolution but the resolving capability is limited to double the input frequency or twice the resolution. Thus, when resolution greater than a factor of two is required, a different type of system is needed in order to provide the higher resolving capability in addition to more sensitive indication of incremental parameter changes.
Accordingly, there exists a need for a system which converts substantially periodic signals into measured parameters and has an improved resolution which can provide a more sensitive indication of incremental changes in the parameter being measured.